A solenoidal coil-shaped conductor is covered with an insulator such as polyurethane. A coil-shaped conductor is formed by winding a conductor together with a tubular insulator such that the tubular insulator is placed between turns of the conductor.
Sheets of conductor and sheets of insulator are disposed alternately.
Related arts are disclosed in Japanese Laid-open Patent Publication Nos. 10-308315, 06-276706, 06-120063, 09-219326, 09-219324, 2008-186848, or 2000-260618.